The present invention relates to endoprostheses for joining live bone stubs and more particularly to a novel ceramic implant for bridging over a static defect between live bone ends without requiring the use of bone cement while providing low mechanical stress on the bone surface and encouraging natural bone growth.
It is known to produce implants and endoprostheses of ceramic materials, such as high purity aluminum oxide and the like. Unlike the other conventional implant materials, such as metal compounds or high-polymeric organic plastics and the like, a ceramic material has a permanently stable chemical state, i.e., implants of a ceramic material show no corrosion or disintegration in the living organism due to chemical decomposition by action of body fluids, even after prolonged periods of implantation. High-purity aluminum oxide has the highest degree of tissue compatibility of all the implant materials known at present. It has already been suggested that a multipart hip joint prostheses be provided with a ball and socket formed of aluminum oxide, according to German Patent 1,912,130. A two-part endoprostheses consisting of a metal shaft having a ball of aluminum oxide and a mating socket also of aluminum oxide is described in German published application No. 2,134,316.
The presently known conventional endoprostheses solutions have disadvantages in that they can only be fixed in place by application of bone cement, or as in the above-mentioned German published application 2,134,316, by new natural bone growth, provided the surface of the implant has a porous layer. Both fixing methods have decided disadvantages in that bone cement causes irreversible changes in the adjoining tissue by the exothermic polymerization reaction and by the resulting chemical products, which can lead to inflammation and bond loosening. Additionally, a very large intermedullar free space must be provided for receiving both the prostheses shaft and the bone cement when this technique is used with a tubular bone. This requirement leads to considerable mechanical and biological change along a very great length of the marrow space, due to implant interference with the essential blood supply vessels leading from the marrow space over the spongy resin to the cortical periphery of the bone.
A difficulty encountered in the application of implants having a porous layer is that the strength of the porous surface layer is not sufficient for the mechanical loads subsequently applied thereto. Aggravating the mechanical aspects of intramedullar fixing is the fact that the marrow space has a relatively small diameter, so that considerable force acts on the bone wall over a relatively short shaft diameter having a relatively low bending moment; these forces increase as the shaft of the implant, introduced into the marrow space, becomes shorter.
German patent application P2 305 329.4-35 describes a holding fixture or sleeve designed to prevent splintering or other damage to the bone, but requiring that an intramedullar shaft portion be utilized. The present invention is an additional improvement thereon, which prevents the occurrence of the above-mentioned mechanical and biological problems.